Memory devices are widely used in computers and many electronic items to store information. A memory device usually has numerous memory cells. The memory device performs a write operation to store information in the memory cells, a read operation to retrieve the stored information, and an erase operation to clear information (e.g., obsolete information) from some or all of the memory cells. The reliability of these operations depends greatly on the structure of the memory device and the techniques used to operate it. Some conventional memory devices have structures and operations that are reliable for some applications but unsuitable for other applications.